The invention relates to an energy supply system with an electricity generating device for the regenerative generation of electrical energy that can be fed into an electricity supply grid, and to an operating method for such an energy supply system.
The use of renewable forms of energy such as wind power, solar energy and hydroelectric power is becoming more and more important for electricity generation. Electrical energy is typically delivered to multiple consumers via long-range, trans-regional and trans-nationally connected electricity supply grids, known as electricity grids for short. As electrical energy as such cannot be stored in significant quantities, the electrical power fed into the electricity grid must be adapted to the consumer-side power demand, the so-called load. It is known that the load fluctuates over time, in particular depending on the time of day, the day of the week or even the season. The load profile is classically divided into three ranges—base load, medium load and peak load—and appropriate electrical energy generators are used depending on the type in these three load ranges. Continuous synchronization of electricity generation and electricity consumption is required for a stable and reliable electricity supply. Any deviations that occur are compensated by so-called positive or negative balancing energy or balancing power. Positive balancing power is required when the normal amount of electricity provided lags too far behind the current electricity demand, in order to prevent an undesired lowering of the grid frequency and failure of the electricity supply caused thereby. Negative balancing power is required when unexpected excess electricity generation power occurs occasioning an unexpected rise in frequency. The problem exists with regenerative electricity generating devices that for certain types such as wind power and solar energy the energy generation power is not present at all times and cannot always be controlled in a certain way, but is subject to fluctuations, e.g. depending on the time of day and the weather, which can only be predicted to a limited degree.
Laid-open publication DE 10 2009 007 567 A1 discloses a method for producing methanol by recycling carbon dioxide from the waste gas of fossil fuel-fired power plants, combined heat and power plants or other emitters of CO2, the CO2 being subjected to a methanol synthesis with hydrogen which is preferably generated from an electrolysis with regeneratively obtained electrical energy, in particular in low load phases of an associated electricity grid. The synthesized methanol can be stored temporarily in a methanol store or conveyed as a fuel to a heat or electricity generating plant. An energy generating installation carrying out the method consists of a combined heat and power plant, a wind, hydroelectric and/or solar power plant, an electrolysis plant, a store for CO2, O2 and H2, respectively, a methanol synthesizing plant, a methanol store and a control system to control these installation components for generating energy depending on the electricity demand in a manner that is optimized for the load.
Laid-open publication DE 43 32 789 A1 discloses a method for storing hydrogen energy by reacting hydrogen, obtained for example by using solar or nuclear energy, in the presence of carbon dioxide into methane or methanol which can then be used, for example, as a fuel for vehicles or combustion plants.
Laid-open publication DE 10 2004 030 717 A1 discloses a method and a device for converting and storing regeneratively obtained energy by converting it into chemical energy with the use of electrical energy and carbon dioxide, the chemical energy being released again as chemical and electrical energy depending on demand. For this purpose, a recirculation process is provided in which energy from a geothermal or regenerative source is converted into electrical energy which is supplied to a consumer and an electrolysis device. Some of the hydrogen obtained from the electrolysis is supplied to a consumer and some is subjected to a synthesis with CO2 from a CO2 store to form a hydrocarbon and an alcohol. The hydrocarbon, for example methane, is stored in an associated store and some of it is supplied to a consumer and some to a combustion heating process to which, on the other hand, oxygen from the electrolysis is supplied. Using a thermodynamic process, the combustion heating process generates electrical energy, some of which is supplied to the electrical consumer and some to the electrolysis process. CO2 generated in the combustion heating process is stored in the same way as CO2 which comes from a CO2 recovery process which is fed with CO2 from the hydrocarbon consumer.
Laid-open publication WO 2009/019159 A2 discloses an energy distribution method with which an electricity grid that distributes electricity or loads is to be operated in a stable manner, regeneratively generated energy being used at least in a significant proportion to generate hydrogen, carbon dioxide from other power plants or a final storage site being hydrogenated together with the hydrogen in a hydrogenation plant, and a gaseous combustible hydrocarbon so generated being recycled in a power plant for generating electricity, wherein carbon is circulated.
It is known for gas supply grids that a supplementary gas and/or a replacement gas can be added to the gas that is usually distributed in a supply area, the so-called base gas, such as natural gas.
Supplementary gases are understood to be gas mixtures which differ significantly from the base gas in their composition and burning characteristics but, when mixed with the base gas, have a similar burning behavior as the base gas, and which are allowed to be added to the base gas in limited quantities. Replacement gases are understood to be gas mixtures which, despite a composition which differs from that of the base gas and burning characteristics which may also differ, have a similar burning behavior as the base gas at the same gas pressure and can completely replace the base gas when necessary. So-called conditioning gases can additionally be added. For the corresponding definitions and classifications of supplementary gas, replacement gas and gases for conditioning, reference can be made to the relevant specialist literature, see for example DVGW Technische Regel Arbeitsblatt G260: Gasbeschaffenheit, Wirtschafts-und Verlagsgesellschaft Gas und Wasser mbH, Bonn, January 2000; DVGW Technische Regel Arbeitsblatt G262: Nutzung von Gasen aus regenerativen Quellen in der öffentlichen Gasversorgung, Wirtschafts-und Verlagsgesellschaft Gas und Wasser mbH, Bonn, November 2004; Gaswärme-Institut e.V. Essen et al: Analyse und Bewertung der Nutzungsmöglichkeiten von Biomasse, Volume 4, 2005; and the study published by the Fachagentur nachwachsende Rohstoffe e.V.: Einspeisung von Biogas in das Erdgasnetz, Leipzig, 2006, ISBN 3-00-018346-9, see in particular pages 68 to 70.
The technical problem which the invention addresses is the provision of an energy supply system of the type mentioned at the beginning and of an operating method suitable therefor, with which regeneratively generated electrical energy can be used in an optimal fashion in order to ensure a stable and reliable electricity supply with a high proportion of electricity from renewable forms of energy even when regenerative electricity generators are used which have a significantly fluctuating generating power, such as wind power and/or photovoltaic power plants.
The invention solves this problem by providing an energy supply system, and an associated operating method.
The energy supply system according to the invention contains, in addition to the regenerative electricity generating device, a hydrogen generating device which can generate hydrogen using the regeneratively generated electrical energy, and a methanation device which can convert the hydrogen generated into a gas containing methane in the presence of a supplied carbon oxide gas, preferably carbon dioxide, or a synthesis gas. Furthermore, the energy supply system contains a gas providing device by means of which a gas can be provided in the quality of a supplementary gas or a replacement gas which can be fed into a gas supply grid, with the use of the gas containing methane from the methanation device and/or the hydrogen from the hydrogen generating device. By means of these installation components, the energy supply system is characteristically capable of providing large quantities of a supplementary gas or replacement gas, which can be fed into a gas supply grid and stored there or used for another purpose, from regeneratively generated electrical energy. Installations known per se can be used for the said system components, such as an electrolysis unit for generating hydrogen and a conventional methanation reactor.
The gas providing device is specially designed so as to provide the gas in a variably specifiable supplementary/replacement gas quality, for which it has, if necessary, corresponding gas preparation means. As a result of this special design of the gas providing device, the gas can be prepared optimally as appropriate for the situation in the quality of a supplementary gas, in order to feed the latter, for example, in a limited quantity into a gas supply grid, or in the quality of a replacement gas, in order to feed the latter, for example, as a full-value replacement for the base gas into a gas supply grid. The preparation of gas in the quality of supplementary gas typically requires a lower energy usage than the preparation of a replacement gas. On the other hand there is no limitation on the feeding of the replacement gas into the gas supply grid as it may optionally completely replace the base gas. It is to be understood that the gas providing device including its associated control means can, as necessary, take the form of a stand-alone installation component or can be integrated wholly or partially into the methanation device and/or the hydrogen generating device and/or a central installation control system. In each case, the gas generated using regenerative electrical energy can thus very advantageously, depending on demand, be provided differently and variably as supplementary or replacement gas and used, for example, for feeding into a gas supply grid.
In an advantageous development of the invention, the energy supply system contains a gas-into-electricity conversion device which can generate electrical energy using gas from the gas supply grid. Suitable means for achieving this are, for example, a high-efficiency gas and steam power plant, called a GuD power plant for short, or another means for converting methane into electricity such as gas turbines or methane-fired block-type thermal power plants having gas Otto engines, or engines or fuel cells which are adapted for methane. The gas and steam power plant will be mentioned primarily below as representative for all options for the conversion of methane into electricity.
The energy supply system thus allows the shifting of regenerative fluctuating electricity generation and hence a greater possibility of using regenerative electricity generators having a generating capacity that fluctuates depending on the type, with the required grid stability of an electricity grid fed with this electricity being maintained. At periods when regeneratively generated electricity, for example from wind power and/or photovoltaic power plants cannot be fed into the electricity grid for grid stability reasons or other technical and commercial reasons, it can be used to generate hydrogen which is transformed with carbon oxide gas by means of methanation into a supplementary or replacement gas which can be fed into the gas supply grid, stored there and used, in particular also for reconversion into electricity at periods with a high residual load of the electricity grid, i.e. with a high difference between the power demand on the electricity grid and the power provided by the regenerative electricity generating device.
The invention thus very efficiently solves the problems of the impossibility of storing sufficiently high amounts of electrical energy, as are required for public electricity grids, of the power output by important regenerative electricity generators which fluctuates, for example, according to the time of day, the season and the weather, and the required grid stability of the electricity grid. The energy supply system according to the invention enables electricity and gas supply grids to be networked or connected, which allows a high loading of regenerative electricity generators by using the prevailing conventional gas storage capacities in the gas supply grid, and additionally can provide high-quality regenerative positive and negative balancing energy or stored regenerative electricity for the electricity grid by appropriate load and generation management.
In an advantageous embodiment of the invention, the electricity generating device comprises in particular one or more wind power plants and/or one or more photovoltaic power plants and/or one or more geothermal power plants and/or one or more biomass power plants and/or one or more hydroelectric power plants and/or one or more solar power plants. In the future, these types of installation will become increasingly important for supplying electrical energy. Their specific integration into the energy supply system according to the invention allows a high degree of utilization thereof with the electricity grid stability maintained despite the power output by some of these regenerative electricity generators fluctuating considerably over time and the demand for electricity which fluctuates over time.
In a development of the invention, the energy supply system has means for supplying carbon dioxide to the methanation device from a power plant outputting carbon dioxide. This allows carbon dioxide which occurs in a power plant to be used and integrated, via the methanation device, into the energy cycle of the energy supply system.
In a development of the invention, the energy supply system contains means for supplying a gas containing carbon dioxide to the methanation device from a biogas installation, such as biogas or carbon dioxide separated therefrom. As a result, the regenerative use of biomass from a conventional biogas plant can advantageously be integrated into the energy supply system according to the invention.
In a development of the invention, according to the invention means are provided for the transfer of heat from the methanation device to a biogas installation. In this way, waste heat from the methanation device can be used in the biogas installation.
In a development of the invention, means are provided for supplying a supplementary or replacement gas obtained from a biogas plant to the gas providing device or to the gas supply grid, i.e. the biogas plant in this case also directly contributes to the feeding of the gas supply grid.
In a further advantageous development of the invention, the energy supply system contains means for supplying a synthesis gas containing carbon oxide from a biomass gasification installation to the methanation device. The synthesis gas, together with the hydrogen, can be converted into the methane-containing supplementary or replacement gas through an appropriate design of the methanation device.
In a development of the invention, the energy supply system contains an ORC (organic Rankine cycle) unit or another heat utilization unit for generating electrical energy using waste heat from the methanation device so that this waste heat can be used directly to generate electricity.
In a development of the invention, the energy supply system comprises a fuel dispensing device by means of which corresponding vehicles can be supplied with hydrogen generated by the hydrogen generating device and/or with electrical energy generated by the regenerative electricity generating device and/or with supplementary or replacement gas delivered by the gas providing device, wherein all three or only some of these energy sources for refuelling vehicles can be provided at the fuel dispensing device depending on the application and needs.
In a development of the invention, the energy supply system comprises an electricity supply grid to which the regenerative electricity generating device and the hydrogen generating device and the gas-into-electricity conversion device are connected, and/or a gas supply grid to which the methanation device and the gas-into-electricity conversion device are connected. The electricity supply grid and/or the gas supply grid are thus integrated into the energy supply system according to the invention, preferably both of them specifically connected to each other. In an embodiment of this aspect of the invention, the energy supply device comprises a gas store connected to the gas supply grid. The gas store can hereby be a gas store known per se for this purpose which is capable of temporarily storing the substitute natural gas delivered by the methanation device with high capacity.
In a development of the invention, the energy supply system comprises a control device which is configured so that the power output of the hydrogen generating device and/or of the gas-into-electricity conversion device can be set variably depending on a time-dependent power demand on the electricity supply grid. This forms the basis for advantageous and efficient load and generation management of the system and in particular of the electricity supply grid.
In an embodiment of this aspect of the invention, the control device is designed so that, at periods when there is an increased demand on the electricity grid for power generation, a reduced power consumption of the hydrogen generating device and/or an increased feed-in power of the gas-into-electricity conversion device can be set and/or, at periods when there is a reduced demand for power generation, the power consumption of the hydrogen generating device can be increased and/or the feed-in power of the gas-into-electricity conversion device can be reduced. In other words, the control device can provide positive and negative balancing energy to ensure grid stability of the electricity grid using the regeneratively generated electrical energy temporarily stored in reconvertible fashion in the gas supply grid as hydrogen and/or a substitute natural gas. This is made possible by operating the system using the method according to the invention.
In another embodiment of this aspect of the invention, the control device is designed so that the power of the hydrogen generating device and/or of the gas-into-electricity conversion device can be set depending on a projected power demand profile of the electricity supply grid and/or a projected electricity generating power profile of the regenerative electricity generating device. This design of the control device allows the energy supply system to operate in a forward-looking manner from the point of view of the required electricity grid stability and taking into consideration the power, which may fluctuate, provided by the regenerative electricity generating device and the fluctuating electricity demand. To do this, the energy supply system can be operated using the operating method according to the invention in a corresponding further development.
In a development of the invention, the supplementary or replacement gas is provided selectively in one of multiple specifiable quality levels. This allows optimal adaptation of the operation of the methanation device and/or of the hydrogen generating device to requirements which can, for example, be set for the supplementary/replacement gas by the gas supply grid. When allowed by the gas supply grid, a supplementary gas of a lower quality level can be provided at lower cost, and when there are higher requirements a supplementary or replacement gas of a higher quality level can be provided at typically a slightly higher cost.
Advantageous embodiments of the invention are described below and illustrated in the drawings.